The present invention relates generally to gas turbine engines, and, more specifically, to combustors therein.
A turbofan gas turbine engine includes in serial flow communication a fan, compressor, combustor, and high and low pressure turbines which power the compressor and fan, respectively. Air is pressurized in the compressor and mixed with fuel in the combustor and ignited for generated hot combustion gases which rotate the turbines to power the fan and compressor.
In powering an aircraft in flight, the turbofan engine is operated at various power levels between idle and maximum power, with an intermediate cruise power level therebetween. Combustor performance affects the entire design of the engine. The combustor must be suitably sized for obtaining the desired maximum power from the engine. The combustion gases discharged to the turbines must have suitable circumferential and radial temperature distributions quantified by conventional pattern and profile factors.
Combustor performance is also controlled for minimizing undesirable exhaust emissions such as smoke, unburned hydrocarbons, carbon monoxide (CO), and nitrogen oxides (NOx). The levels of these exhaust emissions are directly affected by the fuel to air ratio of operation of the combustor from lean, to stoichiometric, and to rich. Tradeoffs in the relative amounts of these exhaust emissions are commonly required over the various power settings of the engine.
Furthermore, the combustor must also be designed for preventing lean blowout, as well as permitting altitude re-starting when necessary.
Accordingly, modem combustor design requires consideration of various competing objectives to provide stable operation of the combustor over various power levels, while minimizing undesirable exhaust emissions.
One turbofan aircraft engine enjoying commercial success is the CF34-3A engine manufactured and sold for many years in this country by the General Electric Company. This engine includes a single annular combustor having film cooled outer and inner liners. The liners include two rows of primary and secondary dilution holes which affect exhaust emissions and pattern and profile factors. The dilution holes cooperate with a single row of carburetors each having a central fuel injector in a swirl cup mounted in an annular dome at the forward ends of the liners.
The dilution holes are configured with two primary holes per swirl cup and four secondary holes per cup. Half of the primary holes are axially aligned with the centerlines of corresponding swirl cups, and half the primary holes are circumferentially offset at the mid-cup position between adjacent cups.
The four secondary dilution holes per cup include a large hole of maximum size coaxially aligned with a corresponding primary hole and swirl cup. Three remaining secondary holes per cup have a uniform small size, with one hole being aligned at the mid-up circumferential position, and the two remaining small holes being aligned at the 1/4 and 3/4+L circumferential positions.
In this way, the hot streaks associated with the corresponding swirl cups are diluted with air from the corresponding primary and secondary dilution holes axially aligned therewith. The additional secondary dilution holes further control the introduction of dilution air for obtaining a generally uniform circumferential pattern factor at the combustor outlet.
The outer primary holes are larger than the inner primary holes, with the small secondary holes of the outer liner being smaller than the small secondary holes of the inner liner. The large secondary holes of both liners have the same size. In this way, the differently sized primary and secondary holes in the outer and inner liners control dilution air introduction into the combustor for additionally controlling the radial profile factor at the combustor outlet.
In the continued development of the CF34 turbofan engine, a larger combustor is desired for increasing the output power from the engine. However, a larger combustor cannot merely be scaled up version of the -3A combustor in view of the interdependence of the combustor components, combustion gases, and cooling and dilution air.
Furthermore, the larger combustor has additional burning volume for permitting re-starting at higher altitudes than the -3A combustor. Yet further, more stringent exhaust emission requirements are required for the larger combustor.
Since the larger combustor is capable of producing more combustion gases, an increase in corresponding exhaust emissions is a design concern. In particular, the risk of increased NOx emissions is a significant design factor which must be evaluated in conjunction with the overall performance of the larger combustor.
Accordingly, it is desired to provide the larger combustor with improved dilution air for limiting increase in exhaust emissions, such as NOx, while obtaining acceptable combustor performance.